How do you test cerebrospinal fluid (CSF)?

How to Test Cerebrospinal Fluid (CSF)

Standard CSF Testing Panel

All CSF samples should undergo a core panel of tests including cell count with differential, glucose and protein concentrations, Gram stain, and bacterial culture, with additional specialized testing determined by clinical suspicion. 1, 2

Basic Tests (Performed on All Samples)

• Cell count with differential: Essential for identifying inflammatory patterns; bacterial meningitis typically shows ≥2,000 WBCs/μL or ≥1,180 neutrophils/μL, while viral infections show lymphocytic pleocytosis (5-1,000 cells/μL) 1, 2

• Glucose measurement: Normal CSF glucose is >35 mg/dL with CSF-to-blood glucose ratio >0.23; lower values strongly suggest bacterial infection 1, 2

• Protein concentration: Normal is <220 mg/dL; elevated levels indicate infection or inflammation 1, 2

• Gram stain and bacterial culture: Critical for identifying causative organisms and guiding antibiotic therapy 1

• Opening pressure: When measured, normal pressure (<5 WBCs/μL with normal protein) essentially excludes meningitis in immunocompetent patients 1, 2

Volume Requirements and Processing

Collect at least 5 mL of CSF for standard testing, with 8-10 mL required for specialized panels, and process within 30 minutes to prevent cellular degradation. 3

• The first tube collected has highest contamination risk and should not be sent for microbiology studies 1

• Larger volumes (5-10 mL) increase sensitivity for mycobacterial and fungal cultures 1

• Fresh samples must be processed within 30-60 minutes; alternatively, fix with ethanol/Carbowax (1:1 ratio) if immediate processing is impossible 3

• Insufficient volume is a common cause of false-negative results 3

Specialized Testing Based on Clinical Context

For Suspected Bacterial Meningitis

• Gram stain, culture, and antimicrobial susceptibility testing 2
• CSF lactate measurement may help distinguish bacterial from aseptic meningitis in neurosurgical patients 1

For Immunocompromised Patients

• Cryptococcal antigen testing 1
• Fungal stains and cultures 1
• Acid-fast bacillus smears and cultures for tuberculosis 1
• PCR testing for herpes simplex virus, CMV, JC virus, West Nile virus, adenovirus, and enterovirus 1, 2

For Suspected Viral Encephalitis/Meningitis

• PCR for herpes simplex virus, enterovirus, and other viruses based on epidemiology 2

For Suspected Neurosyphilis

• CSF VDRL or RPR testing 1, 2

For Suspected Malignancy

• Cytologic examination: Large-volume CSF (minimum 5 mL) improves sensitivity for leptomeningeal carcinomatosis 1, 3
• Consider second lumbar puncture if first result is negative or equivocal 1
• Emerging liquid biopsy techniques (circulating tumor DNA and circulating tumor cells) show enhanced diagnostic sensitivity compared to conventional cytology 1

For Demyelinating Disorders

• Oligoclonal bands, IgG index, and IgG synthesis rate for suspected multiple sclerosis 2

For Neurodegenerative Disease

• Aβ42 and tau/phosphorylated tau profiles for suspected Alzheimer's disease when diagnostic uncertainty remains after imaging 2

Special Considerations for Patients with Intracranial Devices

When patients with ventriculostomy catheters or shunt systems develop fever, obtain CSF from the device reservoir; if CSF flow is obstructed, sample both the device and lumbar space. 1, 2

• Remove ventriculostomy catheters in patients who develop stupor or meningitis signs, and culture the catheter tip 1, 2

• Aspirate CSF reservoirs in patients with shunt systems or Ommaya reservoirs 1, 2

Critical Pitfalls to Avoid

• Hemorrhagic contamination interferes with test interpretation; use proper collection technique 3

• Delayed processing beyond 30 minutes causes cellular degradation and affects results 3

• Insufficient volume collection is a leading cause of false-negative results 3

• Delaying antibiotics for imaging: If bacterial meningitis is suspected and lumbar puncture is delayed for any reason (including CT scan), start empirical antibiotics for rapidly fatal pathogens like S. pneumoniae immediately after obtaining blood cultures 1

• In immunocompromised patients, maintain high suspicion for infection regardless of cell count and glucose concentration until cultures are finalized 1